1. Field of the Invention
The present invention relates to an interference canceller and, more particularly, to a cross polarization interference canceller.
In a digital radio transmission system for transmitting independent dual data by using cross polarizations, such as vertical (V) and horizontal (H) polarizations, of the same channel, an XPIC (cross polarization interference canceller) utilizing a transversal-type filter is generally used as a means for compensating XPD (cross polarization discrimination), which deteriorates in the transmission path or circuit elements.
The XPIC of the present invention compensates for cross polarization interference and does itself cause deterioration of XPD.
2. Description of the Related Art
FIG. 1 is an example of a transversal-type equalizer of the prior art. In FIG. 1, an analog type transversal equalizer used in a multi-level QAM system is illustrated. An intersymbol interference element for the main signal (not illustrated) of channels I and Q of the horizontal polarization is obtained at the output terminal O, and the initial waveform, not including interference distortion, can be recovered by removing the intersymbol interference element from the main signal.
As shown in FIG. 1, the transversal equalizer of the prior art provides, for the channel I, cascade-connected delay lines 21-1 through 21-6 having a delay time, and constant multipliers 22-1 through 22-7 connected before or after the delay lines. Also provided, for the channel Q, are delay lines 23-1 through 23-6 and constant multipliers 24-1 through 24-7.
The constant multipliers 22-1 through 22-7 and 24-1 through 24-7 respectively receive constant factors C.sub.-3 through C.sub.3 and D.sub.-3 through D.sub.3 from a control circuit 25 and output tap outputs for equalization to an adder 26. The adder 26 analogously adds the tap outputs and outputs the result to the output terminal O.
Generally, the practical constant multiplier may be formed as a variable attenuator having a phase inverting function with a dynamic range of about 40-60 dB. It is given the phase information depending on positive or negative control voltage. The practical constant multiplier is also given the attenuating information by an absolute value of the control voltage of the control circuit 25.
Each constant multiplier obtains the necessary tap output by receiving a constant of tap from the control circuit 25, but also outputs a constant residual deterioration element even if the main signal is not deteriorated. Since the individual residual deterioration elements from each constant multiplier are combined in the adder 26, the resulting total residual deterioration element is so large that it cannot be neglected. Particularly when a passing delay time difference of fading to be equalized becomes large, since a number of constant multipliers increases and the constant multipliers are required for both channels I and Q in the QAM, the total residual deterioration element becomes significant due to the number of constant multipliers in use. Therefore, a problem arises in that the main signal receives, even when the signal is not deteriorated, more intensified fixed deterioration from the transversal-type equalizer within the cross polarization interference canceller.
For instance, if it is supposed that a constant multiplier has a dynamic range of 50 dB, a transversal type equalizer of 5 taps provides a D/U (desired/undesired) ratio of 40 dB. However, an equalizer of 7 taps provides a D/U ratio of 39 dB, and an equalizer of 9 taps provides a D/U ratio of 38 dB. Quite clearly, as the number of taps increases, the D/U ratio deteriorates.